Calculating inverter for motor

[Quang_Lo]

Dear All,
I have a problem. It's calculating inverter for 3 pharse non-synchronous. I mean that i don't know the formula. My colleagues show
Here we go :
P[SUB]1[/SUB]/P[SUB]2[/SUB] = (n[SUB]1[/SUB]/n[SUB]2[/SUB])[SUP]3

[/SUP]Please help me out
Thanks

 

[Darkwood]

Occasionally our choice of inverter will be driven by a requirement for a wider range of carrier frequencies than normal.

Frequency of stop/start cycling would be an issue that we'd address with uprated forced ventilation rather than oversizing.

Thats why I used the wording 'excessive' and 'may warrant', as you show other solutions exist but situation dependent as to best solution, I always check with tech' for best solution for my set-up.

 

[Darkwood]

Don't understand how you would get excessive start up current with a correctly designed and set up system.

Not saying you cannot, but just cannot see it in my experience of moving heavy loads with VSD's.

I experienced it with high gear ratio sets with very short ramp times where drive dectects instantaneous over-current or shock load on motor during acceleration or deceleration, in some cases the monitoring circuits of the drives are very effective and can be too keen and when ncreasing ramp times or voltage boosting is not working the manufacturers recommend upgrading to larger drive.

This is a ramping related issue and I know of 2 drives that can give this problem, both tech' support suggest larger drives, I believe there is a fine balance between the trade off of protected outputs against short circuits and trip logic set-ups .... the better output protection you get the more chances that some arrangements may give a false trip.

 

[SheffSimon]

Chokes would sort that out.

Tried chokes, they didn't make an appreciable difference.

 

[Besoeker]

Tried chokes, they didn't make an appreciable difference.

Appreciable difference to what exactly? What problem were you trying to fix?

If it's charging currents that are deemed to be the problem, and that's not an uncommon problem with PWM inverters, the fast switching edges that cause it are there regardless of the inverter rating. Oversizing the inverter won't change that.

You know the i=Cdv/dt thing.

Here's a measurement I took on a system where the fast edges were causing untimely motor failures:

The major divisions on the horizontal axis are at 2us intervals and the minor ones at 0.4us.

We are currently in the build process of a couple of VSDs that have a 300m cable run. We had the option of going for du/dt filters or sine wave filters. There was a significant cost difference which of course we highlighted. The customer opted for the latter.

In both the real life examples I have given, an increase in drive rating was never a consideration as a fix for cable length.
In fact, in my fairly long experience, it never has been.

But, as ever, I'm prepared to learn even if I am beyond my sell-by date.

 

[Quang_Lo]

Dear All
To conclusion, what do you mean ? what is last fomular ?